: A detailed understanding of the structural and functional aspects of the molecules involved in the interaction, binding, and fusion of the HIV-1 virus with its target cells is fundamental to an understanding of the disease process in AIDS and to the rational design of vaccines and other therapeutic and diagnostic agents. The one molecule that is arguably the most important in the entire infectious process, namely the envelope glycoprotein, gpl20, remains the least well characterized. This proposal describes investigations aimed at determining the 3-D morphology of the HIV-1 envelope glycoproteins, gpl20, and its precursor, gpl60. This will be accomplished by determining its overall shape and generating a 3-D epitope and reactive site map of the surface of the molecule. Two forms of high resolution electron microscopy will be used to analyze the envelope glycoproteins alone and in combination with various ligands and monoclonal antibodies to give a characterization of domain organization and a 3-D surface map of epitopes and reactive sites. Negative stain electron microscopy will yield images of molecules and immune complexes as they adhere to a carbon substrate. This is a well established technique for analyzing the structure of macromolecules though it is rarely used for detailed analysis of molecules as small as gpl20. The second approach will be the use of cryo-electron microscopy where unstained molecules are viewed suspended in ice. Analysis of such small molecules may be at the cutting edge of this technology. Extensive use will be made of previously defined and well characterized reagents. The information generated may: 1) reveal the overall domain configuration, 2) give an indication of the intramolecular (segmental) flexibility, and 3) reveal the topological locations of immunological, functional, and sequence-defined reactive sites and epitopes. With this information, it may be possible to propose more detailed mechanistic explanations for envelope glycoprotein function and for the ability of some antibodies but not others to neutralize HIV-1.